The geological time scale encompasses the entire history of the Earth. The geological time of the planet is divided and distributed in intervals of time characterized by important events in the history of the Earth and of life. It is framed over approximately 4.567 million years, in which the first accreted materials in the solar system were dated, giving the age of the earth at 4.54 Ga, at the beginning of the Hadean Eon (not officially recognized). . At the end of the scale, the present day is taken as included in the Holocene Quaternary.
When talking about geological time, it is almost always expressed in millions of years and always referred to "before the present". The units used to divide geological time are of two types: those referring to relative time (geochronological units), which order geological events chronologically, and those referring to absolute time (geochronometric units), expressed in absolute values, in millions of years. .
Geochronological units are units of time based on chronostratigraphic units. Chronostratigraphic units divide the Earth's rocks in chronological order, reflecting the main geological, biological and climatic events that have occurred over time. The names of the chronostratigraphic units share the same name with their geochronological equivalents, except that the names derived from their relative stratigraphic position—lower, middle, and upper—are translated as early, middle, and late. For example, the Upper Cretaceous series is equivalent to the Late Cretaceous epoch.
The geochronological units correspond one by one with the chronostratigraphic ones and are ordered, in descending order of hierarchy, as follows: eon, era, period, epoch, age and chron.
Since it has been possible to date rocks with absolute values (in figures expressed in millions of years), dating of the limits of geochronological units has been adjusted with some precision, depending on the methods used. All the geochronological units —and therefore their chronostratigraphic equivalents— for which their limits have been able to be specified also become geochronometric units. In practice, the geochronometric character of these units is not usually expressed, erroneously implying that the value in years corresponds to geochronological units.
For Precambrian times most units are exclusively geochronometric, and have been defined by more or less arbitrary internationally agreed limits of time.
The methods can be relative, which resort to the ordering of materials in time according to their position in the terrestrial environment, by the principle of the superimposition of strata, or absolute, based on dating by radioactive isotopes, which give measurements in millions of years.
Geologic Time Scale
The geological time scale is the reference framework to represent the events of the history of the Earth and of life ordered chronologically. It establishes divisions and subdivisions of the rocks according to their relative age and the absolute time elapsed since the formation of the Earth to the present.
Geological time scale
age of the earth
Cohen, K.M.; Finney, S. & Gibbard, P.L. (2012). «International Chronostratigraphic Chart» (in English). International Stratigraphic Commission. Archived from the original on October 30, 2012. Retrieved November 10, 2012.